1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 + TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU02/4%#4)/.G0G0!).34G0G0).4%2&%2%.#%#!,#5,!4)/.G0G0/&G0G06/,4!%G0G0).$5#%$G0G0).4/4%,%#/-5.)#!4)/.G0G0,).%3G0G0&2/-G02!$)/34!4)/.G0G0“2/!$#!343G0G0!.$G0G0-%4(/$3G0G0/&2%$5#).G0G0).4%2&%2%.#%)45G134G0G0Recommen
2、dationG0G0+ (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation K.18 was published in Volume IX of the Blue Book. This file is an extract from theBlue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identica
3、l to the Blue Book version and copyright conditions remain unchanged (see below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this
4、publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Volume IX - Rec. K.18 1Recommendation K.18Volume IX - Rec. K.18CALCULATION OF VOLTAGE INDUCED INTO TELECOMMUNICATIONLINES F
5、ROM RADIO STATION BROADCASTS AND METHODSOF REDUCING INTERFERENCEGeneva, 1980, modified at Malaga-Torremolinos, 1984 and at Melbourne, 1988)1 IntroductionAlthough inductive interference from radio waves is seldom observed on circuits in underground cables, manyexamples of such interference have been
6、reported in circuits carried by open wires, aerial cables or cables inside buildings.Interference on voice-frequency circuits occurs because the induced radio wave is detected and demodulated by thenonlinear components in a telephone set or by metal oxide layers formed at conductor joints. This inte
7、rference is mostlyintelligible noise and may occur up to 5 km from a radio station whose radiating power is more than several tens ofkilowatts.On carrier or video transmission circuits, the induced radio wave impairs circuit performance when the radio-wavefrequency is within the operating frequency
8、of the transmission system. The interference usually consists of a singlefrequency tone within a telephone channel and is unintelligible. It reduces the signal-to-noise ratio (SNR) for thetransmission system. This interference may occur within a wide area around a radio station. Interference on vide
9、otransmission circuits has been reported in only a few cases, but it is expected to cause serious problems when videotransmission services increase in number in the future.An unusual example of interference may arise in which outside plant maintenance personnel receive burns due toradio frequency cu
10、rrents. Such problems have been reported only in the immediate vicinity of a radio station antenna.2 Analysis of interferenceIn the theoretical analysis of the voltage induced from a radio wave, the following conditions are assumed: Earth resistivity is homogeneous and uniform. A cable or a wire is
11、supported in a straight line at a constant height above the earths surface. The metallic screen of a cable is earthed at both ends. The radio-wave electric field has a constant intensity and a constant incidence angle, and phase change along thecable is uniform. The radio wave is originally polarize
12、d vertically. However, while it propagates along the surface of the earth, ahorizontal component is generated due to the finite conductivity of the earth.Constants and variables used for theoretical analysis are shown in Annex A.2.1 For telecommunication lines without a metallic screen, the horizont
13、al component of the radio-wave electric field actsdirectly as an electromotive force on the telecommunication line. This causes induced noise at terminals when the circuit hasan impedance unbalance with respect to earth. Induced longitudinal voltages at the ends of a telecommunication line withouta
14、metallic screen are given by Equations (B-1) and (B-2).2.2 For telecommunication cables with a metallic screen, the horizontal component of the radio-wave electric field actsas an electromotive force, causing induced current to flow in the earth return circuit composed of the metallic screen of thec
15、able and the earth. Due to the current in the screen, an electromotive force is induced in the conductors through the transferimpedance between the conductors and the metallic screen. This electromotive force may cause disturbance to metalliccircuits in the cable, according to the degree of their un
16、balance with respect to the metallic screen (or the earth).Induced longitudinal voltages at the ends of a telecommunication cable with a metallic screen are given by Equations(B-3) and (B-4). In reference 1 the values obtained by using these equations are shown to agree with measured values.2 Volume
17、 IX - Rec. K.182.3 The equations in Annex B are very complicated and involve many parameters. It is therefore useful to estimate theapproximate value of the maximum induced longitudinal voltage by the following simplified equation:( )()()VVl VPE ZZfvK22 2012020430 20 3001010 10 10(0) dB log 020 log
18、cos log log (2-1)=+wherelfZZ Zf(2 - 2)(2 - 3)j(dB / km)1R 1L=+=15 1020100280122 22203., 20is the attenuation coefficient at 1 MHz (dB/km)f is the radio-wave frequency expressed in Hz.Other constants and variables are shown in Annex A.Equation (2-1), which gives the maximum induced longitudinal volta
19、ge in dB (0 dB = 0.775 V), is obtained on thebasis of the following:The induced longitudinal voltage calculated by the equations in Annex B reaches an initial peak value when cablelengthlf=15 10028. and subsequently describes a series of peak values. Its maximum value occurs at one of the earlies pe
20、ak values along thecable length.lf15 10028.The induced longitudinal voltage reaches its maximum at one of the earliest peak values due to the attenuation of theinduced radio wave along the cable (Figure 3/K.18).The errors involved in using Equation (2-1) instead of the full equations of Annex B are
21、described in detail in AnnexC.2.4 If the line configuration is very complicated, it is necessary to divide the line into several segments and to estimatethe induced longitudinal voltage for each segment by Equations (B-1) to (B-4). Estimated induced voltages for each segmentare then combined to obta
22、in the overall induced voltage, taking into account the transmission characteristics and theboundary conditions of the line involved.When the simplified equation (2-1) is applied to a complicated line, a straight line model may be used to estimate themaximum induced longitudinal voltage. Calculation
23、s should commence at the point nearest to the radio station and thesmallest value of radio wave incidence angle should be used.2.5 When field measurement of the radio-wave electric field strength is carried out, the measured value may be used forEvin Equation (2-1).When the measured value is not ava
24、ilable, the radio-wave electric field strength Evcan be calculated by Equation (2-4), taking into account the distance from the radio station and the power of the radio station transmitter (see 2).Volume IX - Rec. K.18 3ErPZv= 1(2 - 4)15 20.piwhereP is the radio station transmitting power (W)r is th
25、e distance from radio station (m)Z0is the instrinsic impedance of free space ( 377 )Figure 1/K.18 shows values of Evobtained from Equation (2-4) using various values of P.2.6 The angle of incidence made by the radio wave onto the telecommunication line may vary according tocircumstances.When the tel
26、ecommunication line is installed in open country, either a measured value of the incidence angle or avalue calculated from the relative location of the radio station and the telecommunication line may be used.When the telecommunication line is installed near structures which obstruct radio wave prop
27、agation, the incidenceangle may be taken as zero and the severest condition assumed.2.7 The induced longitudinal voltage at the ends of the telecommunication cable shown in Figure 2/K.18 may beestimated using the simplified method which follows.Inserting the values for parameters P, f, 20, 2and give
28、n in Figure 2/K.18 together with calculated values for Evand ZKinto Equations (2-1) and (2-2), the following results are obtained:V2 (0) V2 (l) = 35.0 dBl 210 m4 Volume IX - Rec. K.18Moreover, using = 0 as the most severe value, the following is obtained:V2 (0) V2 (l) = 32.0 dBl 210 mIn Figure 3/K.1
29、8 the results obtained by using the simplified calculations are compared with others derived fromusing the more rigorous methods described in Annex B, in which values of V2related to cable length are expressed. It isapparent that the simplified method is adequate for estimating the most severe inter
30、ference likely to be experienced.Volume IX - Rec. K.18 52.8 Transverse voltages which cause noise arise due to the imperfect balance of the circuit with respect to the metallicscreen (or earth). If a ratio, is used to related longitudinal and transverse voltages, noise levels may be obtained fromcal
31、culated or measured values of the induced longitudinal voltage:V = V2whereV2 V2(0) or V2(l) is the longitudinal voltage at the ends of the longitudinal circuit under open circuit conditions,V V(0) or V(l) is the transverse voltage at the ends of the circuit when terminated with its characteristic im
32、pedanceat both ends.For example, in the case shown in Figure 2/K.18 and equal to 40 dB, the noise level, V is obtained as follows:(in this case, V2= 35 dB 0 dB = 0.775 V)V = 35 40 dB = 75 dB3 Reduction of interferenceThe following measures may be taken to minimize interference:3.1 Interference to a
33、voice-frequency circuit can be reduced by inserting a 0.01 0.05 F capacitor between conductorsand the earth at the input terminal or at the telephone set, to bypass induced radio-wave currents.3.2 Interference to carrier and video transmission systems can be reduced by the following measures:6 Volum
34、e IX - Rec. K.183.2.1 An adequate screen should be incorporated in the cable, e.g. a 0.2 mm thick aluminium screen around a cableprovides a reduction of interference of about 70 dB. The aluminium screen should be earthed at both ends with resistanceless than Z01 , when earth conductivity is less tha
35、n 0.1 S/m. If the screen thickness is increased to 1.0 mm the reductionis improved by a further 50-60 dB.3.2.2 Conductors should be completely shielded by a metallic screen around cable joints and at cable terminals.Note If the metallic screen is removed for a length of about 30 cm, induced voltages
36、 increase by about 30 dB, evenif the metallic screen is connected electrically. Even if only 5 cm of the metallic screen is removed from a cable end, inducedvoltages increase by about 10 dB.3.2.3 In sections susceptible to radio-wave interference, underground cable should be installed or different c
37、able routingsshould be used.3.2.4 Distances between repeaters should be reduced to provide an acceptable signal-to-noise ratio (SNR) for the system.3.2.5 The admittance unbalance of the terminal equipment and repeaters at the radio-wave frequency should be improvedwith respect to earth.3.2.6 Pre-emp
38、hasized level setting of the transmission system should be used.3.3 To reduce the induced dangerous voltage to mainteance personnel, a capacitor may be inserted between theconductors and the earth at suitable intervals within the induced section to bypass the induced current.In this case, care must
39、be taken, in selecting an appropriate capacitor, to combine minimum attenuation of thetransmission frequencies with effective earthing at the radio-wave frequency. Care should be taken to prevent the capacitorfrom being damaged by overvoltages appearing on the conductors.ANNEX A(to Recommendation K.
40、18)Constants and variables used in Recommendation K.18A.1 The ratio of horizontal component to vertical component, P for a radio-wave electric field propagating along theground surface is:PEEhvr= = 1j (A 1)0whereEhis the horizontal component in radio wave electric field strength (V/m)Evis the vertic
41、al component in radio wave electric field strength (V/m)ris the specific dielectric constant of earth0is the specific dielectric constant of free space (F/m)Z0is the intrinsic impedance of free space ()0is the phase constant of free space (rad/m) is the earth conductivity (S/m) is the angular freque
42、ncy of radio wave (rad/s)f is the frequency of radio wave (Hz)Volume IX - Rec. K.18 7A.2 The transfer impedance of the metallic screen of a cable sheath, ZKis:ZKtKtRK=sinh dc/m (A-2)whereRdcis the direct-current resistance per unit length of metallic screen (/m)K = jg is the permeability of metallic
43、 screen (H/m)g is the conductivity of metallic screen (S/m)t is the thickness of metallic screen (m).A.3 In connection with the following symbols, see Figure A-1/K.18. is the incidence angle of radio wave to telecommunication line (rad)l is the cable length (m)x is the distance along the cable from
44、the cable end near to the radio station (meters)Z01is the earth return circuit characteristic impedance ()1is the earth return circuit propagation constantZ02is the longitudinal circuit characteristic impedance ()2is the longitudinal circuit propagation constantZ1L, Z1Rearth return circuit terminal
45、impedance ()Z2L, Z2Rlongitudinal circuit terminal impedance ()101 101 1LLL=+ZZZZis the earth return circuit current reflection coefficient at x = 0101 101 1RRR=+ZZZZis the earth return circuit current reflection coefficient at x = l202 202 2LLL=+ZZZZis the longitudinal circuit current reflection at
46、x = 0202 202 2RRR=+ZZZZis the longitudinal circuit current reflection at x = lV1m(x) (for m = 0) is the voltage in earth return circuit with matching at both endsV1m(x) (for m = L) is the voltage in earth return circuit with mismatching at x = 0V1m(x) (for m = R) is the voltage in earth return circu
47、it with mismatching at x = lV2m(x) (for m = 0) is the voltage in longitudinal circuit with matching at both endsV2m(x) (for m = L) is the voltage in longitudinal circuit with mismatching at x = 08 Volume IX - Rec. K.18V2m(x) (for m = R) is the voltage in longitudinal circuit with mismatching at x =
48、lANNEX B(to Recommendation K.18)Induced longitudinal voltage calculationB.1 Telecommunication lines without metallic screenInduced longitudinal voltages at the ends of a telecommunication line without a metallic screen are given byEquations (B-1) and (B-2).Induced longitudinal voltage at the end nea
49、rest the radio station:VV V VPEVVlvlllll1101 1122102101111(0) (0) (0) (0) V (0) cos 21 e + j cos V (0) 1 ee(0) V (0) e 1 e() LR10( + j cos ) 1L1L 1R1L 1R1R1R 1L1L 1R1011=+= (B-1)Volume IX - Rec. K.18 9Induced longitudinal voltage at the end farthest from the radio station:Vl V l V l V lVlPEVl VVlvllll1101 11011111101112111111( )
